Generally speaking, diagnostic imaging employing ultrasonic waves is performed by 2-dimensional B-mode imaging. With 2-dimensional B-mode imaging, the object body under diagnosis is irradiated by ultrasonic waves, and the resultant ultrasonic echo waves are received and converted into electrical signals, and processed to form a 2-dimensional image. The image primarily shows the spatial distribution of the echo source intensity within the body under diagnosis. This image is displayed on a display apparatus such as a CRT. Other types of diagnostic imaging technique include M-mode imaging as well as the pulsed Doppler technique. By means of the pulsed Doppler technique detecting the Doppler effect, movement within the object body under diagnosis (for example, movement of the heart, blood flow in the blood vessels, etc.) can be examined.
However, using the pulsed Doppler technique, it is essential to employ an imaging apparatus whereby the sampling points for the pulsed Doppler system can be set by the operator while the operator observes the 2-dimensional B-mode image in real time. In this way, the positioning of the sampling points from which Doppler data is to be derived, and hence the positions of the corresponding echo source, can be performed by the operator so as to track continuously or in step to the regions of interest within the object body under diagnosis.
In the prior art of diagnostic imaging apparatus, it has been difficult to perform time sharing data acquisition in real time for both the 2-dimensional B-mode imaging operation and the pulsed Doppler operation, without mutual interference in between. Hereinafter, the respective modes of these two operations will be referred to as the B and D modes. More specifically, if it is attempted, with such a prior art type of diagnostic imaging apparatus, to fully exploit the real time operation capability of 2-dimensional B- mode imaging, it is necessary to allocate part of the pulse echo sequences to the B-mode and another sequence to the pulsed Doppler mode. Here, the pulse echo sequences are to be determined so as to have maximum available pulse repetition rate for each given depth of penetration, for example, about 4 kHz for body scanning. Among these pulse echo sequences, half of them might be allocated to the B mode and the other half to the pulsed Doppler mode. In this way, each mode is successively executed in half of entire pulse sequences. With this method the data rate for the pulsed Doppler mode is one half of the rate for the case where the entire pulse sequences are to the pulsed Doppler mode. Thus, the maximum detectable Doppler shift will be extremely low if observation is to be performed such that no Doppler ambiguity will arise. In other words, in such a case, arterial blood flow signal of the normal subject may already get too high Doppler shift for such system to cause its spectrum to reach the folded back region. in the spectrum display. Thus, it is practically impossible to carry out proper diagnosis for .even higher Doppler shift region, which means a considerable limitation in practice.
Conversely, in order to overcome this problem, if entire pulse echo sequences are dedicated to the pulsed Doppler mode, then it becomes impossible to obtain a 2-dimensional B-mode image to determine the sample point position. This will cause erroneous reading by the operator. It might be suggested that it would be allowed to perform B-mode operation for extremely brief duration to interrupt the execution of the pulsed Doppler mode, i.e. once or twice per second, to momentarily enter the B-mode operation. In this case, if for example, the pulsed Doppler mode is halted for a duration of 20 to 40 milliseconds per second, there will be no significant adverse effect upon observation in the pulsed Doppler mode. However, when this method is employed, it would still be very difficult to determine the positions of the sampling point and the sound beam for pulsed Doppler mode by dynamically following the 2-dimensional B-mode image. It has been considered that this type of problem is unavoidable, if a single echo sounder (having the function of receiving and beam forming the ultrasonic waves) is employed for both the B and D modes in time sharing manner.
A proposal to overcome the above problem has been given in Japanese Patent Laid Open No. 58-89242. With the proposed method using a single beam, during each second of pulsed Doppler mode execution, interruption for of B-mode scanning to form a complete field, or for a part of a field, are repetitively performed at a sufficiently high rate for obtaining a 2-dimensional image having acceptable real time characteristics. During such interruption the Doppler signal is lost for a short time and interpolation for such lost signal in the Doppler signal is performed based on measurement of that signal during rest of time where no B-mode interruption is executed. However, with this method, it is necessary to perform interpolation of the Doppler signal over a comparatively long time interval, for example, 15 milliseconds, for interruption of B-mode operation. The method and means to perform such compensation are complex, and it also is necessary to employ a signal editing means to have continuity between the compensated portions of the Doppler signal and the directly measured portion of the same.